As a result of their increased performance in terms of output, specific power and power density, synchronous machines with permanent magnets have an extensive application nowadays in the field of motor vehicles.
These electrical machines can be produced in a wide range of powers and speeds, and have applications both in vehicles of the all-electric type, and in vehicles with low CO2 emission of the types known as mild-hybrid and full-hybrid.
The mild-hybrid applications generally concern electrical machines of approximately 8 to 20 kW, for example an electric motor fitted on the front face of a thermal engine, and coupled to the latter by a drive belt. With an electric motor of this type, it is possible to reduce the cubic capacity of the thermal motorisation (engine downsizing) by providing electric torque assistance which supplies additional power, in particular during restarting. In addition, traction at low speed, for example in an urban environment, can also be ensured by this same electric motor.
Applications of the full-hybrid type generally concern 30 to 50 kW motors for architectures of the series and/or parallel type, with a level of integration which is more complete than that of the electric motor(s) in the traction chain of the vehicle.
Machines with rare earth permanent magnets, such as magnets of the neodymium-iron-boron (NeFeB), samarium-iron (SmFe) or samarium-cobalt (SmCo) type have remarkable performance in terms of magnetic flux, since the rare earth magnets can have remanences which exceed a tesla.
However, machines with permanent magnets comprising a rotor with a so-called “flux concentration” structure have made it possible to obtain substantial magnetic fluxes with magnets with lower remanence, for example magnets obtained from sintered or bonded ferrites.
Since the implementation of rare earth magnets in a rotor of an electrical machine which is designed for motor vehicle applications is no longer economically viable, the other alternative thus consists of magnets based on ferrites.
However, since the remanence or induction of ferrite is lower than in the case of a rare earth magnet, it has been necessary to increase the volume of the ferrite magnet in order to obtain an equivalent magnetic flux. In patent application FR2982093, there has thus been proposed a rotor with permanent magnets with flux concentration having partly a trapezoidal form which makes it possible to increase the volume of the magnets in the rotor.
However, it is still necessary to optimise the dimensional and magnetic characteristics of this type of rotor, such as to improve the magnetic flux, whilst complying with dimensional and mechanical constraints specified.